1. Field of the Invention
This invention relates to a foundation having a cylindrical shell for supporting a superstructure on the foundation, and a construction method therefor. In particular, the invention relates to a foundation having a cylindrical shell, which will be suitable for supporting structures, towers, tanks, silos, piers of bridges, etc., and a construction method therefor wherein the foundation is constructed leaving the soil inside the cylindrical shell, i.e., without discharging the soil.
2. Description of the Background Art
Well foundations or open caisson foundations are well known as cylindrical foundations, and such foundations are disclosed in U.S. Pat. Nos. 3,618,327 and 3,939,664. Such foundations are constituted by vertically placing a cylindrical structure, the upper and lower ends of which are open, in the place of installation, causing the cylindrical structure to settle into the ground while excavating the soil at the portion surrounded by the cylindrical structure, and finally placing a bottom slab of concrete. However, the size of the foundation itself has become greater recently with the increase in the scale of a superstructure placed on the foundation, and the diameter of a well of the well foundation also has become greater. Also, the excavation quantity of the internal soil has become greater during the excavation, and the volume of jobs for the excavation and discharge of the soil and the cost of the work accompanied thereby has increased remarkably.
The recent progress in the excavation technology has made it possible to excavate easily a deep groove in the ground, and the technology of an in-situ concrete diaphragm wall has made it possible to form a cylindrical structure. Therefore, a cylindrical foundation capable of being executed without excavating and discharging the soil thereinside such as in the conventional well foundation has been developed, such as the cylindrical foundation disclosed in Japanese Patent Laid-Open No. 186009/1990.
In the cylindrical foundation of the type described above, however, the soil inside the cylindrical structure, which is to serve as the shell, is not excavated and discharged but is left as is described above. If a superstructure is built up on such a cylindrical foundation, as the remaining soil gradually undergoes consolidation settlement, the negative friction resulting from this consolidation settling exerts adverse influences of adding a load bearing to the cylindrical structure. Furthermore, a cavity is created at the upper part on the inside of the cylindrical structure, so that flowing water remains inside the foundation and invites the breakage of the concrete due to freezing and the corrosion of reinforcing bars disposed at this part of the cylindrical structure.
Technology to mix and stir a hardener, such as a cement milk, with the soil to form a solidified soil layer and thereby constituting an underground structure without excavating and discharging the soil in situ is known as disclosed in U.S. Pat. Nos. 4,886,400 and 4,906,142. The underground structure consisting of this solidified soil provides an advantage over ordinary concrete structures in that it can be constituted without excavating and discharging the soil in situ, but yet involves problems such as a reduced reliability of strength and an increased possibility of degradation resulting from external environmental factors. Therefore, such an underground structure has mainly been used as a temporary retaining wall or a cut-off wall rather than as the foundation structure itself.